JP2009081230A - Surface-emitting laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-emitting laser having capability of high speed operation and excellent reliability. <P>SOLUTION: The surface-emitting laser includes a semiconductor substrate 101, a first reflector 102 formed on the semiconductor substrate 101, an active layer 103 formed on the first reflector 102, a tunnel junction layer 105 formed on the active layer 103, a first conductivity type semiconductor spacer layer 107 covering the tunnel junction layer 105, a second reflector 108 formed in a region right above the tunnel junction layer 105 on the semiconductor spacer layer 107, a first electrode 110 formed in the peripheral part of the second reflector 108 on the semiconductor spacer layer 107, and a second electrode 111 electrically connected to a layer lower than the active layer 103. The thickness of the first conductivity type semiconductor spacer layer 107 in the region right above the tunnel junction layer 105 is thinner than the thickness of the first conductivity type semiconductor spacer layer 107 in the region right below the first electrode 110. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a surface emitting laser, and more particularly to a tunnel junction type surface emitting laser element.

  2. Description of the Related Art In recent years, vertical cavity surface emitting lasers (VCSELs) having advantages such as high speed, small size, low power consumption, and low cost have been actively developed as light sources for optical communication. An element capable of high-speed modulation of about 1 to 10 Gbps (gigabit per second) has already been put into practical use.

  The most common configuration of a surface emitting laser is an oxide constriction type based on a GaAs substrate. This oxidized constriction type has a laminated structure in which an active layer and semiconductor Bragg reflectors (DBR: Distributed Bragg reflectors) positioned above and below the active layer are integrally formed on a GaAs substrate by epitaxial growth. Moreover, it has a current confinement structure formed by a steam oxidation process. Such an element generally has a structure in which upper and lower DBRs are formed of a semiconductor and current is injected into an active layer through the DBR. In addition, the formation of the current confinement structure by steam oxidation has a feature that the manufacturing process is simple, such as no need for burying regrowth.

  On the other hand, among surface emitting lasers, a tunnel junction type using a tunnel junction as a current confinement structure can achieve high speed and low power consumption by reducing resistance. This tunnel junction type typically has a structure reported in Non-Patent Document 1, is conventionally based on an InP substrate, and has a long wavelength such as an oscillation wavelength of 1.3 μm band or 1.55 μm band for telecom applications. Used in band surface emitting lasers.

  The inventors have used a strained quantum well having a well layer of InGaAs with excellent gain characteristics as a base layer based on a GaAs substrate mainly used for oxidation confinement, and a tunnel using a tunnel junction structure as a current confinement structure. A junction type surface emitting laser has been developed, and a 3 GHz modulation band of 24 GHz, which exceeds that of an oxidized confined type surface emitting laser, has been achieved. The element structure of this tunnel junction type surface emitting laser is disclosed in FIG.

  The element structure of the tunnel junction type surface emitting laser described in Non-Patent Document 2 will be described with reference to FIG. In the surface emitting laser of FIG. 6, a lower DBR 102, an active layer 103 having a quantum well structure, a p-type spacer layer 104, a tunnel junction layer 105, and an n-type spacer layer 107 are formed on a semiconductor substrate 101 made of n-type GaAs. It has a semiconductor stacked structure. Each layer on the n-type semiconductor substrate 101 is sequentially formed by epitaxial growth. The tunnel junction layer 105 includes a high concentration p-type InGaAs layer and a high concentration n-type GaAsSb layer.

  The tunnel junction layer 105 has a structure in which a peripheral region other than a region that finally becomes a light emitting portion is removed by etching in the substrate plane, and then the whole is buried with an n-type spacer layer 107. In order to operate the device at high speed, the high resistance portion 106 is formed by ion implantation around the current injection region A1 before the n-type spacer layer 107 is formed. Thereby, the element capacitance can be reduced. An upper DBR 108, a positive electrode 110, a negative electrode 111, and a polyimide layer 109 are formed on the semiconductor stacked structure. Current is injected into the active layer 103 via the plus electrode 110 and the minus electrode 111, and laser oscillation and high-speed modulation operation are performed.

  The tunnel junction type surface emitting laser described in Non-Patent Document 2 is extremely promising as a light source that is high speed, low cost, and low power consumption, but has a problem in reliability. FIG. 7 shows acceleration reliability test data of the tunnel junction type surface emitting laser. FIG. 7A shows the transition of the light output fluctuation when an energization acceleration test is performed at a constant current drive at a temperature of 150 ° C. and 10 mA. The number of test elements is nine. In each element, the light output suddenly decreases after a predetermined time.

FIG. 7B is a graph showing the relationship between the failure time (Time to Failure: TTF = operation time until failure) of the test element of FIG. 7A and the element structure. Failure time of each element, there is a clear correlation with the L _ELEC in distance or 6 of the open end of the tunnel junction layer 105 and the (end of the current injection region) and the positive electrode 110, the more time to failure L _ELEC is long It is getting longer.

  As a result of investigating the faulty element, it was found that the alloy front of the positive electrode 110 penetrates the n-type spacer layer 107 and reaches the active layer 103. In the tunnel junction surface emitting laser described in Non-Patent Document 2, the thickness of the n-type spacer layer 107 is 0.23 μm, and the distance between the active layer 103 and the plus electrode 110 (D1 = D2) is 0.29 μm. Degree. The plus electrode 110 is made of AuGe / AuGeNi formed by vacuum deposition and alloy processes. Lattice defects introduced into the active layer 103 by the alloying process of the positive electrode 110 are gradually extended toward the current injection region A1 by energization, and when this is reached, the light output suddenly decreases. It was.

In order to prevent the above failure, it is conceivable that L _ELEC is made sufficiently long. However, when L _ELEC is made long, the series resistance from the plus electrode 110 to the tunnel junction layer 105 increases. Therefore, in reality, L _ELEC is limited to about 5 μm or less. Therefore, lengthening L _ELEC is not an essential remedy for the failure.
It is also considered effective to make the n-type spacer layer 107 sufficiently thick so that the alloy front of the positive electrode 110 does not reach the active layer 103. However, simply increasing the thickness of the n-type spacer layer 107 increases the distance L _CAVITY between the upper and lower DBRs. Therefore, there is a problem that the effective resonator length becomes long and the high-speed performance is impaired.

  The problems in the tunnel junction type surface emitting laser described above have not been recognized so far. In a general oxidation confined surface emitting laser, the upper DBR layer is made of a p-type semiconductor, and the upper electrode is made of non-alloyed Ti / Au or the like. For this reason, it is not necessary to consider the progress of the alloy by energization. Further, even if the alloy progresses or the upper DBR layer is made of an n-type semiconductor and the alloy electrode is used as the upper electrode, the thickness between the upper electrode and the active layer 103 is about 3 to 4 μm. Therefore, there is no defect in the active layer 103.

  On the other hand, even in a long wavelength surface emitting laser having an upper DBR formed of a dielectric / semiconductor and having a tunnel junction, this problem has not been recognized for the following three reasons. The first reason is that in an InP-based long-wavelength surface emitting laser, non-alloyed Ti / Au or the like may be used as the n-electrode material, so it is not necessary to consider the influence on the reliability due to the progress of the alloy front .

  The second reason is due to the semiconductor material. In a long-wavelength surface emitting laser, InGaAs having a small band gap can be used as a semiconductor layer in electrical contact with the upper n-electrode. InP is used for the n-type spacer layer. Since the growth of crystal defects is suppressed in a semiconductor containing In, even if there is a crystal defect starting from the upper n electrode, the extension of the defect to the active layer is suppressed, and the reliability of the device is reduced. Less likely to have an impact.

The third reason is that long wavelength band surface emitting lasers have not been sufficiently studied to realize high-speed operation exceeding 10 Gbps. This is because the long-wavelength surface emitting laser does not have sufficient element gain due to the characteristics of the material, and it is difficult to expect high-speed performance that originally exceeds 10 Gbps. If the modulation speed is 10 Gbps or less, there is no need to make L _CAVITY sufficiently short in the first place, so the above problem cannot be recognized. Non-patent document 3 will be described later.
Markus Ortsiefer et al., “2.5-mW Single-Mode Operation of 1.55-μm Buried Tunnel Junction VCSELs”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 8, August 2005 Yashiki et al., “1.1 μm-range tunnel junction VCSELs with 27-GHz relaxation oscillation frequency”, Proceedings of optical fiber communication conference 2007, OMK1, 2007 Ye Zhou et al., "Novel Surface Emitting Laser using High-Contrast Subwavelength Grating", Conference Digest of Semiconductor Laser Conference 2006, WB4, 2006

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a surface-emitting laser capable of high-speed operation and excellent in reliability.

  A surface-emitting laser according to the present invention is formed on a semiconductor substrate, a first reflecting mirror formed on the semiconductor substrate, an active layer formed on the first reflecting mirror, and the active layer. A tunnel junction layer; a first conductivity type semiconductor spacer layer covering the tunnel junction layer; and a second conductivity type formed on the first conductivity type semiconductor spacer layer and in a region immediately above the tunnel junction layer. A first electrode formed on the reflector, on the first conductive type semiconductor spacer layer and around the second reflector, and electrically connected to a layer below the active layer A surface-emitting laser including a second electrode, wherein the thickness of the first-conductivity-type semiconductor spacer layer in the region directly above the tunnel junction layer is equal to the first conductivity in the region immediately below the first electrode. It is characterized by being thinner than the thickness of the semiconductor spacer layer of the mold It is an.

  According to the present invention, it is possible to provide a surface emitting laser capable of high-speed operation and excellent in reliability.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, for clarity of explanation, the following description and drawings are simplified as appropriate.

First Embodiment FIG. 1 is a cross-sectional view of a surface emitting laser element according to a first embodiment of the present invention. In the surface emitting laser according to the present embodiment, the lower DBR 102, the quantum well structure active layer 103, the p-type spacer layer 104, the tunnel junction layer 105, and the n-type spacer layer 107 are formed on the semiconductor substrate 101 made of n-type GaAs. It has a formed semiconductor stacked structure. Each layer on the n-type semiconductor substrate 101 is sequentially formed by epitaxial growth. The tunnel junction layer 105 includes a high concentration p-type InGaAs layer and a high concentration n-type GaAsSb layer.

The lower DBR 102 includes, for example, a low refractive index layer made of Si-doped (concentration 8 × 10 ¹⁷ cm ⁻³ ) n-type AlAs and a high refractive index layer made of Si-doped (concentration 8 × 10 ¹⁷ cm ⁻³ ) n-type GaAs. Are stacked in order. Specifically, a periodic structure in which 40.5 pairs of a low refractive index layer and a high refractive index layer, each having an optical film thickness λ / 4, are stacked.

The active layer 103 is, for example, a quantum well layer (the emission spectrum peak is 1.07 μm) using In _0.3 Ga _0.7 As / GaAs as a well layer and GaAs as a barrier layer, and n-type disposed at both ends thereof. An SCH layer and a p-type SCH layer are provided. The optical film thickness of the entire active layer 103 is preferably 1λ (one wavelength within the medium).
The p-type spacer layer 104 is made of, for example, C-doped (concentration 8 × 10 ¹⁷ cm ⁻³ ) p-type GaAs. The optical film thickness is preferably about λ / 4.
Here, in the strained quantum well structure active layer using InGaAs as the well layer, the range of the laser oscillation wavelength suitable for realizing the high-speed modulation operation is 1.0 to about 1.0 to when the active layer is formed on the GaAs substrate. The range is about 1.2 μm. When the oscillation wavelength is less than 1.0 μm, it is difficult to obtain the merit of gain improvement by the strained quantum well structure. On the other hand, when the oscillation wavelength exceeds 1.2 μm, there is a concern that reliability deteriorates because the amount of crystal distortion is too large. However, if an InGaAs ternary substrate having a larger lattice constant than GaAs can be used as the substrate, the oscillation wavelength can be increased to about 1.34 μm while maintaining high reliability. However, when the oscillation wavelength is further increased, the effective cavity length of the surface emitting laser, which will be described in detail later, is increased, which is a disadvantage for high-speed operation.

The tunnel junction layer 105 includes a high concentration p-type InGaAs layer and a high concentration n-type GaAsSb layer. For example, a high concentration p-type layer having a thickness of about 15 nm made of C-doped (concentration 1 × 10 ²⁰ cm ⁻³ ) p-type GaAs and a thickness of 15 nm made of Si-doped (concentration 2 × 10 ¹⁹ cm ⁻³ ) n-type GaAs. A moderately high concentration n-type layer can be used.

  Further, the periphery of the tunnel junction layer 105 is removed by etching in the substrate plane, leaving only the portion that will eventually become the current injection region A1. Further, for high-speed operation of the device, the peripheral portion of the current injection region A1 is designed to reduce the device capacitance by the high resistance portion 106 formed by ion implantation. The entire surface of the semiconductor multilayer structure in which the tunnel junction layer 105 and the high resistance portion 106 are formed is buried with an n-type spacer layer 107.

  Here, current injection into the active layer 103 is performed through the tunnel junction layer 105 in the current injection region A <b> 1, which is a formation region of the tunnel junction layer 105. On the other hand, in the region having no tunnel junction, the n-type spacer layer 107 and the p-type spacer layer 104 are adjacent to each other. Due to this current confinement structure, light emission occurs in the active layer 103 near the current injection region A1.

The n-type spacer layer 107 is made of, for example, Si-doped (concentration 2 × 10 ¹⁹ cm ⁻³ ) n-type GaAs. In the n-type spacer layer 107, a recess 112 is formed immediately above the current injection region A1 and its vicinity. Due to the presence of the recess 112, in the vicinity of the current injection region A1, the distance between the upper surface of the n-type spacer layer 107 and the central surface of the active layer 103 in the direction of the optical resonator is indicated by D1 in FIG. Can be shortened. On the other hand, immediately below the plus electrode 110, this distance can be increased as indicated by D2 in FIG.

Specifically, in the current injection region A1, the distance D1 is preferably equivalent to the optical film thickness _1λ , and the distance L _CAVITY between the upper and lower DBRs is preferably equivalent to the optical film thickness _3λ / 2. As a result, in the current injection region A1 and the vicinity thereof, a short L _CAVITY necessary for high-speed operation of the surface emitting laser can be realized. Note that the optical thickness of L _CAVITY for realizing high-speed modulation exceeding 10 Gbps is preferably about 5λ / 2 or less. The variation in the thickness of the n-type spacer layer 107 in the region where the tunnel junction layer 105 is formed, that is, the current injection region A1, is preferably equal to or less than the optical film thickness λ / 10.

  Further, in the region outside the current injection region A1, the n-type spacer layer 107 is not etched, so that the distance D2 between the positive electrode 110 and the center surface of the active layer 103 is 1.0 μm or more. Is preferred. As a result, a structure in which the front of the alloy in the plus electrode 110 cannot reach the active layer 103 can be realized, so that the reliability problem in the tunnel junction surface emitting laser described in Non-Patent Document 1 can be solved. As described above, according to the present invention, it is possible to achieve both high-speed modulation operation exceeding 20 Gbps and high element reliability of the tunnel junction type surface emitting laser.

  An upper DBR 108, a positive electrode 110, a negative electrode 111, and a polyimide layer 109 are formed on the semiconductor laminated structure after the formation of the recess 112, and laser oscillation and oscillation are performed by current injection into the active layer 103 through the positive electrode 110 and the negative electrode 111. A high speed modulation operation is performed. Here, both the positive electrode 110 and the negative electrode 111 are n electrodes, and are alloy electrodes made of Au / Ge / Ni.

The upper DBR 108 is preferably composed of an SiO ₂ low refractive index layer (thickness 181.2 nm) with an optical film thickness λ / 4 and an Si high refractive index layer (thickness 71.5 nm) with an optical film thickness λ / 4. A three-period layer structure can be formed. As a material for the high refractive index layer, Si, Sb ₂ S ₃ , ZnSe, CdS, ZnS, TiO ₂ or the like can be considered. Further, as a material for the low refractive index layer, SiO ₂ , SiN _x , MgO, CaF ₂ , MgF ₂ , Al ₂ O _{3 and the} like can be considered. An appropriate transparent material is selected in consideration of the oscillation wavelength.

  Note that nitrogen, phosphorus, Te, or the like may be used as an element constituting the semiconductor stacked structure, and the semiconductor substrate 101 is not limited to GaAs, and InP, InGaAs, GaN, or the like may be used.

Next, the characteristics of the tunnel junction type surface emitting laser according to the present invention with respect to the oxidized confined type surface emitting laser will be described. There are two main features. The first feature is that the tunnel junction layer 105 is used as a current confinement structure. The tunnel junction is a high-concentration pn junction. When a reverse bias is applied thereto, the electron current can be converted into a hole current by the tunnel effect. By forming this tunnel junction in the p-type semiconductor layer near the active layer 103, the outermost layer of the semiconductor layer can be an n-type semiconductor. Thereby, reduction of element resistance and absorption loss, suppression of nonuniform injection, etc. can be expected. A second feature is that the upper DBR 108 is formed of a Si / SiO ₂ multilayer film having a high refractive index difference, that is, a high Δ. When a high Δ DBR is used, the effective resonator length of the VCSEL is shortened, so that the modulation band can be improved.

ここで、ｄｇ／ｄｎは微分利得、Ｖｐはレーザ発振光のモード体積である。式（１）よりＶｐが小さいほど、ｆｒが改善されることが分かる。なお、第１の特徴であるトンネル接合構造による素子抵抗や吸収損失の低減などの効果は、素子の自己発熱の抑制につながるため、本式におけるｄｇ／ｄｎの項の改善効果として現れる。 The effect of improving the high speed characteristics by the high Δ DBR will be described below. The modulation band of the direct modulation laser is a balance between a band (f _CR ) determined by the rate limiting of the element resistance (R) and the parasitic capacitance (C) and a band (fr: relaxation oscillation frequency) determined by the gain characteristics of the current injection element. It is determined. In the oxide confinement type VCSEL, for f _CR band, reducing or R by optimizing the semiconductor layer structure by epitaxial growth, to more than 20GHz by taking appropriate measures, such as C reduce by polyimide buried or ion implantation structure It is possible. On the other hand, fr remains at about 16 GHz, and the main rate-limiting factor in realizing high-speed operation of 20 Gbps or more in the oxidized constriction type VCSEL is fr. fr is expressed by the following equation.

Here, dg / dn is the differential gain, and Vp is the mode volume of the laser oscillation light. From the formula (1), it can be seen that as Vp is smaller, fr is improved. The effect of reducing the element resistance and absorption loss due to the tunnel junction structure, which is the first feature, leads to suppression of self-heating of the element, and thus appears as an improvement effect of the dg / dn term in this equation.

In the tunnel junction type surface emitting laser according to the present invention, it was found that fr was improved because the optical resonator direction component in the mode volume Vp was shortened by the upper DBR of high Δ. This will be described below with reference to FIG. 2 (a) and 2 (b) respectively _show an oxide constriction type VCSEL having an optical film thickness of _1λ as a distance between upper and lower DBRs (L _CAVITY ) and a tunnel junction (TJ) type VCSEL having an L _{CAVITY of 3λ} / 2. 2 shows the intensity distribution of the optical field in the direction of the resonator. The horizontal axis represents the position in the thickness direction, and the vertical axis represents the relative light intensity. The plus side of the horizontal axis is the substrate side. The optical field and its envelope are shown as a solid line and a wavy line, respectively. In both structures, a standing wave optical field is formed by upper and lower DBRs sandwiching the active layer 103.

Here, the effective resonator length ( _LEFF ) often used as an index for discussing the mode volume in the direction of the optical resonator in the VCSEL is used. _LEFF is defined as the width of the region where the relative light intensity is 1 / e or more. Here, e is the base of the natural logarithm. Further, a region having a relative light intensity of 1 / e or more includes a portion belonging to the upper DBR (L1 in FIGS. 2A and 2B), a portion belonging to the active layer (L2), and the lower DBR in the stacked structure. As the sum of the parts belonging to (L3).

In the oxidized constriction type VCSEL of FIG. 2A, both the upper DBR and the lower DBR are formed of an AlGaAs / GaAs semiconductor, and the optical field extending widths L1 and L3 to both regions are approximately 467 nm. Also it includes an active layer, the width of the optical field in the resonator portion having an optical thickness of 1λ is 308 nm, _{L EFF} in real space becomes about 1242Nm. The structure of the oxidized constriction type VCSEL shown here is an element which is designed to be capable of high-speed operation at 20 Gbps or more with fr of 16 GHz, a 3 dB modulation band of 20 GHz.

On the other hand, in the tunnel junction type VCSEL of FIG. 2B, the lower DBR is formed of an AlGaAs / GaAs semiconductor as in the case of the oxide constriction type VCSEL. Therefore, the extension width L3 of the optical field to this region is It is about 467 nm. On the other hand, since the upper DBR 108 uses a high Δ multilayer film made of Si / SiO _{2, the} optical field intensity rapidly decreases, and the stretch width L1 is significantly shortened to about 55 nm. Active layer, the width L2 of the light field, which is the optical thickness of the 3 [lambda] / 2 in the resonator portion, including a tunnel junction, longer than oxidized confinement type, is about 463 nm, L _EFF in consequently this device It turns out that it is 984 nm in real space, and is shortened rather than the oxidation confinement type | mold VCSEL. The structure of the tunnel junction type VCSEL shown here is an element having an opening diameter of the tunnel junction portion of 5 μm. At this time, as a fr, a high frequency of 23 GHz that cannot be reached by the oxidized constriction type VCSEL is obtained. Yes. In addition, the 3 dB modulation band is 24 GHz, and high-speed operation at 20 Gbps or higher has been confirmed.

Table 1 summarizes the L _EFF comparison of both structures. Here, for the tunnel junction (TJ) type VCSEL, L _EFF having a structure in which the resonator length L _CAVITY is _increased to 2λ and 5λ / 2 is also shown. From Table 1, also _{L EFF} may be on the order 1293Nm, a length of about equivalent to that of the conventional oxide-confinement VCSEL resonator length _{L CAVITY} is 5 [lambda] / 2 degree and long structure of the tunnel junction type VCSEL, fr = 16 GHz It can be seen that a modulation band of 20 GHz and an operation speed of 20 Gbps or more can be realized.

  As described above, in the tunnel junction type surface emitting laser according to the present invention, since the mode volume can be reduced by the high ΔDBR, the fr can be improved as compared with the oxidized constriction type VCSEL.

Next, a method of manufacturing the surface emitting laser element shown in FIG. 1 will be described with reference to FIGS. 3 (a) to 3 (d) and FIGS. 4 (e) to 4 (g).
First, as shown in FIG. 3A, a semiconductor laminated structure from a lower DBR 102 which is a first reflecting mirror to a tunnel junction layer 105 is formed on an n-type semiconductor substrate 101 by metal organic chemical vapor deposition (MOVPE). : Metal-Organic Vapor Phase Epitaxy). This stacked structure includes at least a lower DBR 102, an active layer 103, a p-type spacer layer 104, and a tunnel junction layer 105. In order to improve device characteristics, an additional semiconductor layer such as a gradient composition layer may be appropriately inserted.

  Next, as shown in FIG. 3B, a resist pattern is formed by photolithography, and the tunnel junction layer 105 other than the current injection region A1 is removed by a known etching means. At this time, the etching depth is preferably about 30 nm. Moreover, the planar shape of the current injection region A1 is not particularly limited, but may be a circle having a diameter of about 3 to 10 μm, for example. After the etching step, the high resistance portion 106 is formed by oxygen ion implantation around the current injection region A1. Here, the high resistance increasing portion 106 is preferably a region outside the diameter of 12 μm with the center of the current injection region A1 as the center.

  Next, as shown in FIG. 3C, an n-type spacer layer 107 made of Si-doped n-type GaAs is formed with a thickness of 0.94 μm by the second crystal growth step. By forming the n-type spacer layer 107, a buried tunnel junction type current blocking structure is formed. The thickness of the n-type spacer layer 107 is desirably set to the minimum necessary within a range in which crystal defects generated on the alloy front due to the formation of the plus electrode 110 and the alloy process do not reach the active layer 103. In our examination, it was confirmed that sufficient reliability can be secured by setting the distance D2 from the active layer 103 to the plus electrode 110 to be approximately 1.0 μm or more. At this time, the thickness of the n-type spacer layer 107 made of GaAs is about 0.94 μm. The material of the n-type spacer layer 107 is changed to a material that is less prone to crystal defects such as InGaAs, InGaP, and InGaAsP to which a small amount of In is added instead of GaAs. It is possible to reduce the required thickness.

  Next, as shown in FIG. 3D, a recess 112 was formed in the n-type spacer layer 107 in and around the current injection region A1 using a known etching means. By this etching process, it is preferable that the thickness of the n-type spacer layer 107 is set to an optical film thickness of λ / 4 immediately above the current injection region A1 and its peripheral portion. The bottom surface region A2 of the recess 112 is formed to be larger by about 0.5 to 6 μm than the diameter of the current injection region A1 so that all the oscillation modes included in the laser oscillation light do not contact the recess 112. It is preferable to do this.

  Next, as shown in FIG. 4E, an upper DBR 108 as a second reflecting mirror is formed on the surface of the n-type spacer layer 107 by sputtering, and then a positive electrode is formed using a photolithography process and known etching means. 110 inner diameter region A3 was left, and the upper DBR 108 in the outer peripheral region was removed. Further, the mesa was formed by removing the semiconductor laminated structure at the outer peripheral portion to a depth reaching the lower DBR 102 by using a photolithography process and known etching means. Here, the diameter of the mesa formation region A4 can be set to 22 μm, for example.

  Thereafter, as shown in FIG. 4F, a polyimide layer 109, which is a structure for reducing the pad capacity of the positive electrode 110 necessary for high-speed operation, is formed by a photolithography process.

  Finally, as shown in FIG. 4G, the positive electrode 110 and the negative electrode 111 made of Au / Ge / Ni are formed, and then electrode alloying is performed, whereby the surface according to the present embodiment shown in FIG. A light emitting laser element is completed. The electrode alloy can be performed, for example, under conditions of a temperature of 375 ° C. and a time of 10 seconds.

  Note that the semiconductor materials and manufacturing methods used in the present invention are not limited to the present embodiment. The upper DBR 108 is formed by sputtering such as RF sputtering or reactive sputtering, electron beam evaporation, CVD (Chemical Vapor Deposition), ion beam assisted deposition, MOVPE, molecular beam epitaxy (MBE). A method such as Molecular Beam Epitaxy) may be used.

  The lower DBR 102 is not limited to the form of the present embodiment. In addition to the semiconductor DBR, a multilayer film made of a semiconductor / dielectric material is used similarly to the upper DBR 108 by removing and depositing the semiconductor substrate 101 by etching. Also good. Further, a DBR composed of a semiconductor / steam oxide film may be formed using a steam oxidation process, or only a steam oxide film is formed with respect to a DBR composed of a semiconductor / steam oxide film formed using this steam oxidation process. A DBR made of a semiconductor / void formed by performing a step of selectively etching the substrate may be used. Further, a reflecting mirror other than the DBR may be used by metal deposition or the like.

  In the case where the upper DBR 108 and the lower DBR 102 are made of semiconductors, a low-refractive index layer having a large band gap and a high-refractive index layer having a small band gap are provided to facilitate current injection and reduce element resistance. A barrier relaxation layer having an intermediate band gap for relaxing the band discontinuity may be introduced therebetween.

Second Embodiment FIG. 5 is a cross-sectional view of a surface emitting laser element according to a second embodiment of the present invention. The element structure of the present embodiment is characterized in that a sub-wavelength diffraction grating 113 is used as the upper reflecting mirror instead of DBR. The sub-wavelength diffraction grating 113 is made of a semiconductor / dielectric material or the like and has a periodic planar structure having a period shorter than the laser oscillation wavelength. As a report example of a surface emitting laser provided with a sub-wavelength diffraction grating as an upper reflecting mirror, for example, Non-Patent Document 3 is cited. The surface emitting laser of this document is composed of AlGaAs and a gap, and a sub-wavelength diffraction grating having a periodic stripe-like planar structure below the oscillation wavelength of the laser is provided in the semiconductor layer structure above the active layer. The upper and lower layers are also composed of voids. In this report, polarization-controlled single-mode laser oscillation is obtained by using a subwavelength diffraction grating.

The structure of the present embodiment is the same as that of the first embodiment except that a sub-wavelength diffraction grating 113 is used instead of DBR as the upper reflecting mirror. In the present embodiment, the sub-wavelength diffraction grating 113 is formed as the step of FIG. 4E described in the first embodiment. That is, a layer made of SiO _{2 having a} thickness of 180 nm and Si having a thickness of 80 nm is laminated on the surface of the semiconductor layer after forming the recess 112 in FIG. Thereafter, a sub-wavelength diffraction grating 113 having a stripe-like periodic structure was formed by electron beam exposure and dry etching. At this time, the stripe-like periodic structure has a structure in which Si with a width of 80 nm are arranged at a pitch of 260 nm. The Si stripe-like periodic structure formed by this process is completed by being embedded with SiO ₂ having a thickness of 360 nm by a sputtering process. Other steps are the same as those in the first embodiment.

From the viewpoint of high-speed operation, the reflecting mirror using the sub-wavelength diffraction grating 113 has a reflectivity of 99% or more, and thus can be made thinner than the reflecting mirror using DBR. Thereby, the effective resonator length _LEFF of the surface emitting laser is shortened, and further improvement of the high-speed modulation characteristics due to an increase in fr can be expected.

  The features of the present invention from the viewpoint of element resistance and heat dissipation will be described. In a related art surface emitting laser using a sub-wavelength diffraction grating as an upper reflecting mirror, an upper electrode is formed on a p-type semiconductor, and current injection from the peripheral upper p electrode to the active layer 103 is a thin p-type spacer. Since it was performed from the lateral direction through the layer 104, there was a problem that spatial hole burning due to non-uniform carrier injection was likely to occur. In addition, an increase in element resistance is a serious problem. In addition, although the amount of heat generation increases as the element resistance increases, the sub-wavelength diffraction grating usually has a large thermal resistance and poor heat dissipation, so the waste heat cannot be efficiently generated. Is a problem. Due to this poor heat dissipation, the active layer temperature rises, causing a reduction in device gain. Due to the above problems, it is extremely difficult to realize a high-speed operation exceeding 20 Gbps by the current injection type surface emitting laser of the related art having the sub-wavelength diffraction grating and the upper p-electrode. In the present embodiment, the sub-wavelength diffraction grating 113 is formed on the n-type spacer layer 107 that has a low element resistance and is unlikely to generate non-uniform carrier injection due to carrier conversion by the tunnel junction layer 105, so that the sub-wavelength It is possible to avoid the problem of increase in element resistance and thermal resistance due to the formation of the diffraction grating 113.

  As described above, according to the tunnel junction type surface emitting laser according to the present embodiment, higher speed characteristics can be expected as compared with the tunnel junction type surface emitting laser using DBR.

  The surface emitting laser element of the present invention can be applied to optical interconnection such as an ultrahigh speed computer.

It is the fragmentary sectional view which showed typically the structure of the surface emitting laser element which concerns on 1st Embodiment. 4 is a graph showing an optical field distribution in the direction of an optical resonator in the surface emitting laser element and the oxidized confined surface emitting laser according to the first embodiment. It is the fragmentary sectional view which showed typically the manufacturing process of the surface emitting laser element which concerns on 1st Embodiment. It is the fragmentary sectional view which showed typically the manufacturing process of the surface emitting laser element which concerns on 1st Embodiment. It is the fragmentary sectional view which showed typically the structure of the surface emitting laser element which concerns on 2nd Embodiment. It is the fragmentary sectional view which showed typically the structure of the surface emitting laser element relevant to this invention. It is an experimental data shown about the reliability of the element relevant to this invention.

Explanation of symbols

101 Semiconductor substrate 102 Lower DBR
103 active layer 104 p-type spacer layer 105 tunnel junction layer 106 high resistance portion 107 n-type spacer layer 108 upper DBR
109 Polyimide layer 110 Plus electrode 111 Negative electrode 112 Concave 113 Subwavelength diffraction grating A1 Current injection region A2 Concave bottom surface region A3 Positive electrode inner diameter region A4 Mesa formation region

Claims (13)

A semiconductor substrate;
A first reflecting mirror formed on the semiconductor substrate;
An active layer formed on the first reflector;
A tunnel junction layer formed on the active layer;
A first conductivity type semiconductor spacer layer covering the tunnel junction layer;
A second reflecting mirror formed on the semiconductor spacer layer of the first conductivity type and in a region immediately above the tunnel junction layer;
A first electrode formed on a semiconductor spacer layer of the first conductivity type and around the second reflecting mirror;
A surface-emitting laser comprising a second electrode electrically connected to a layer below the active layer,
The thickness of the first conductivity type semiconductor spacer layer in a region immediately above the tunnel junction layer is smaller than the thickness of the first conductivity type semiconductor spacer layer in a region immediately below the first electrode. Surface emitting laser.   2. The surface emitting laser according to claim 1, wherein the semiconductor spacer layer of the first conductivity type has a concave shape that is recessed toward the tunnel junction layer in a region immediately above the tunnel junction layer.   The surface emitting laser according to claim 1 or 2, wherein the first electrode is an alloy electrode.   The distance in the direction perpendicular to the semiconductor substrate between the first electrode and the center plane of the active layer is at least 1.0 μm or more. A surface emitting laser according to claim 1.   The distance between the first and second reflecting mirrors in the tunnel junction layer formation region is an optical film thickness of 5λ / 2 (where λ is a laser oscillation wavelength) or less. The surface emitting laser according to any one of the above.   2. The variation in thickness of the first conductivity type semiconductor spacer layer in the formation region of the tunnel junction layer is less than or equal to an optical film thickness λ / 10 (where λ is a laser oscillation wavelength). The surface emitting laser as described in any one of -5.   A distributed Bragg reflector having a laminated structure of a semiconductor layer and a dielectric layer or a periodic structure of voids in the semiconductor layer is used as at least one of the first and second reflectors. Item 7. The surface emitting laser according to any one of Items 1 to 6.   The surface emitting laser according to claim 1, wherein a sub-wavelength diffraction grating is used for at least one of the first and second reflecting mirrors.   The surface emitting laser according to claim 1, further comprising a second conductivity type semiconductor spacer layer between the active layer and the tunnel junction layer.   The surface emitting laser according to any one of claims 1 to 9, wherein the semiconductor substrate is a compound semiconductor mixed crystal substrate containing Ga and As.   The surface emitting laser according to claim 1, wherein a semiconductor layer containing In is included in at least a part of the first conductivity type semiconductor spacer layer.   The surface emitting laser according to any one of claims 1 to 11, wherein the well layer of the semiconductor quantum well structure constituting the active layer is an InGaAs semiconductor mixed crystal.   The surface emitting laser according to claim 1, wherein the oscillation wavelength is 1.0 μm to 1.34 μm.

Images